Unfortunately for those of us with school-aged kids, September doesn't work. That said, I've been there in May and January. Both were fantastic, but Winter in particular is truly magical at YS.
thank you, and yes, I completely understand about the school aged kids.. been there...
Yellowstone concession shut down in September as do some of the roads. Important to check in advance of traveling there. I like to photograph the elk in particular as they go into rut but the lodging is often not available inside the park in September.